MicroRNAs (miRNAs) are small, non-coding RNAs that act as post-transcriptional regulators. MicroRNA deregulation is a common feature of human cancers, and numerous miRNAs have oncogenic or tumor suppressive properties. The miR-34 family (miR-34a, miR-34b and miR-34c) has garnered much attention because it is directly regulated by p53 and can induce cell cycle arrest and apoptosis in vitro. Importantly, its inactivation has been observed in a number of human cancers. Despite the growing body of evidence suggesting its role as a tumor suppressor, most, if not all, previous studies on miR-34 have been done in vitro or using non-physiologic expression levels of miR-34, which are prone to artifactual results. Hence, its exact functions, mechanism and functionally relevant targets in vivo are still largely unknown. These gaps of knowledge prevent its exploitation for therapeutic intervention. The objective of this application is to investigate the physiologic and tumor suppressive properties of the miR-34 family. The central hypothesis is that members of the miR-34 family are important modulators of the p53 response, and are bona fide tumor suppressors. The proposal aims to answer the following questions: 1) Does chronic loss of miR-34 expression promote tumorigenesis in vivo? 2) What is the physiologic role of miR-34 in the p53 pathway? 3) Which p53 targets strongly cooperate with miR-34? To answer these questions, this application proposes to take advantage of constitutive and conditional knockout mice for the miR-34 family previously generated in our laboratory. Complete genetic inactivation of miR-34 function is compatible with viability in mammals, providing a unique opportunity to test the tumor suppressive properties of these miRNAs in a physiologic context. Initial results show that p53-dependent functions in miR-34-deficient mice and cells are intact, suggesting the functions of this miRNA family in the p53 pathway are likely redundant or highly context-specific. Thus, the experiments proposed in the first Aim of this application focus on two mouse models of human cancers wherein miR-34 loss has been described, while experiments under the second Aim seek to uncover redundancies between miR-34 and other p53 targets. The detailed understanding functions of members of the miR-34 family in normal and pathological processes that will emerge from the proposed study is a critical step towards not only exploring their tumor suppressive properties, but also developing novel anticancer strategies.